1. Field of the Invention
This invention relates to a communication device with a LAN (Local Area Network) interface and an ATM (Asynchronous Transfer Mode) interface and relates to a network system with this communication device.
2. Description of the Related Art
Conventionally, some access control methods are proposed to avoid a transmission information collision which occurs in a LAN. As one of typical access control methods, there is a CSMA/CD (Carrier Sense Multiple Access with Collision Detection). In the LAN adopting CSMA/CD, when a collision of transmission information is detected, the transmission information is re-transmitted after waiting a random amount of time.
A proposal is given of a device which has a interface to the LAN adopting this CSMA/CD (such as 10BASE-5, 10BASE-T, 100BASE-TX and Ethernet, hereinafter, called "Ethernet etc.") and an ATM interface, and which performs intercommunication between Ethernet etc. and an ATM network. Namely, a device connecting a LAN such as Ethernet and an ATM network (hereinafter, called "ATM device") is proposed.
FIG. 19 is an explanatory view showing an example of the above-described ATM device. In FIG. 19, an ATM device 1 is provided with a CSMA/CD interface part 2, an up cell buffer 3 connected to the CSMA/CD interface part 2, an ATM-SW part 4 connected to the up cell buffer 3, a down cell buffer 5 connected to the ATM-SW part 4, and a CSMA/CD interface 6 connected to down cell buffer 5. A LAN 7 is connected to the CSMA/CD interface 2 through an Ethernet interface (LAN interface), and a LAN 8 is connected to the CSMA/CD interface 6 through an Ethernet interface. With this arrangement, the so-called ATM-LAN network is structured. Incidentally, FIG. 19 shows the ATM device regarding the LAN 7 as the up side and the LAN 8 as the down side and transmitting data received from the LAN 7 to the LAN 8.
In the ATM-LAN network shown in FIG. 19, when a plurality of Ethernet frames (packet: hereinafter, called "frame") is transmitted from the LAN 7 to the ATM device 1, a frame reception part 2a in the CSMA/CD interface part 2 receives each frame. Each frame received by the frame reception part 2a is stored in a buffer 2b. The frame stored in the buffer 2b is read by a ATM cell transmission part 2c. The ATM cell transmission part 2c takes out data (user data) from the frame read from the buffer 2b, and stores the user data into an ATM cell (hereinafter, called "cell"). Then, the ATM cell transmission part 2c transmits cells to the ATM-SW part 4 in accordance with a predetermined cell transmission quantity. Each cell transmitted from the ATM cell transmission part 2c is stored in the up cell buffer 3.
The ATM-SW part 4 receives a cell from the up cell buffer 3 and executes switching for the cell. The is transmitted to the CSMA/CD interface part 6 hereby. The cell transmitted from the ATM-SW part 4 is stored in the down cell buffer 5. An ATM cell reception part 6a in the CSMA/CD interface part 6 reads a cell from the down cell buffer 5 in accordance with a predetermined reading quantity. Successively, the ATM cell reception part 6a takes out data from the received cell, and stores the data into the frame. Then, the ATM cell reception part 6a stores the frame keeping the data in the buffer 6b. The frame transmission part 6c reads a frame from the buffer 6b, and transmits the frame to the LAN 8. In this way, data communication is executed between the LAN 7 and the LAN 8.
Now, as shown in FIG. 19, for example, when the data transmission rate in the LAN 7 is 100 Mbps and the data transmission rate in the LAN 8 is 10 Mbps, the ATM device 1 can not transmit the data received from the LAN 7 to the LAN 8 at the data transmission rate in the LAN 7. Thus, the ATM device 1 transmits frames to the LAN 8 little by little. In this case, the ATM device 1 prevents the loss of data received from the LAN 7 by storing data (cell) received from the LAN 7 in the cell buffer 5. However, for example, when the ATM device 1 continuously receives a large quantity of data (frame) from the LAN 7, the cell buffer 5 gets impossible to keep cells and then the so-called buffer overflow (overflow) occurs in the cell buffer 5, whereby there is a case in that data is lost.
In view of this problem, in the ATM device 1, a not-shown control unit contained in the ATM-SW part 4 monitors the data storage quantity in the down cell buffer 5. When the data storage quantity in the down cell buffer 5 exceeds a predetermined threshold, the not-shown control unit transmits a band control signal to the ATM cell transmission part 2c (refer to FIG. 20). The ATM cell transmission part 2c, when receiving the band control signal, lowers the cell transmission quantity. As a result, the quantity of cells transmitted from the ATM-SW part 4 to the down cell buffer 5 lowers, therefore, the overflow in the down cell buffer 5 is prevented.
The above-described ATM device 1, however, has the following problem. For example, as shown in FIG. 21, it is assumed that there is a ATM-LAN network in which a LAN 8 or LAN 8a is connected to each of ATM devices 1a, 1b structured similarly to the ATM device 1 shown in FIG. 19, and these ATM devices 1a, 1b are connected to the ATM network 9. In this ATM-LAN network, each of the ATM devices 1a, 1b can control only the down cell buffer 5 in each of the ATM devices. That is, the ATM device 1acannot control the down cell buffer 5 in the ATM device 1b, and the ATM device 1b cannot control the down cell buffer 5 in the ATM device 1a.
Accordingly, as shown in FIG. 21, when a large quantity of data is transmitted from the LAN 8a to the LAN 8, there is a possibility in that overflows of the downward buffer in the ATM device 1a occurs, however, the ATM device 1a cannot stop the cells transmitted the from ATM device 1b. Thus, there is a possibility in that overflow occurs in the downward buffer 5 of the ATM device 1b.
Further, in the ATM device 1 shown in FIG. 19, as shown in FIG. 22, the ATM transmission part 2c in the CSMA/CD interface part 2 receives a band control signal and lowers the transmission band of cells (transmission rate) according to the band control signal, thereby preventing overflow in the downward buffer 5. However, when the transmission rate in the ATM cell transmission part 2c is narrowed, the frame rate of the upward buffer 2b rises, and, at last, frames overflow from the upward buffer 2b.
When frames overflow from the upward buffer 2b, the frames are discarded. However, it is not immediately notified to the LAN 7 that frames are discarded. In this case, frames received from the LAN 7 are checked by the high layer in another LAN to be a communicated partner of the LAN 7. An error caused by not receiving discarded frames is detected, and the error is recognized to the LAN 7, whereby the LAN 7 know for the first time that frames are discarded.
Accordingly, to restore the data communication between the LAN 7 and the other into the normal condition, there is only a way in that the re-transmission procedure by the high layer is executed between the LAN 7 and the other LAN. However, it takes a lot of time till the re-transmission procedure of frame is finished after an error is detected on the basis of the frame discard in the other LAN. Thus, there is a possibility in that throughput of data communication deteriorates.
Further, when the storage quantity of buffer is controlled without considering property (such as half duplex line, CSMA/CD method) of the LAN (Ethernet) connected to the ATM device 1, the data transmission quantity between LANs is narrowed needlessly, whereby there is a possibility in that the throughput lowers.